Classically, the resolution of a racemic mixture into enantiomers involves conversion of the racemate into a mixture of diastereomers. The conversion requires use of an optically active reagent. Diastereomers have more than one chiral center and are not mirror images. Thus, they have different achiral properties such as solubility, enabling them to be separated.
The majority of resolutions that have been carried out involve the reaction of organic bases with organic acids to yield salts. For example, when a practitioner wishes to resolve a racemic mixture of an organic acid, he or she reacts the mixture with an optically active base, such as quinine or the like, to prepare the corresponding mixture of salts. The salts in the mixture will have different properties, including solubility. Therefore, they can be separated by fractional crystallization. After the separation has taken place, optically active acid can be recovered from each salt by reaction with strong mineral acid, which displaces the weaker organic acid. If the salt had been purified by repeated recrystallizations to remove traces of the other diastereomer, the acid will be recovered in optically pure form.
Resolution of organic bases is conducted by a reversal of the process. In these instances, the reaction of an optically active acid with the racemic base is followed by repeated fractional crystallizations to separate the diastereomeric salts obtained. Reaction of a strong base with the purified salt displaces the weaker organic base employed in the salt formation.
The resolution of alcohols poses a difficult problem. Alcohols are not strong acids or bases. Hence they cannot be resolved directly by salt formation. To overcome this difficulty, the alcohols are chemically attached to an acid which can form a salt. After the acid is no longer needed, it is removed. Such a resolution process is difficult, expensive, and tedious. Hence, there is a need for improved methods for the resolution of racemic mixtures of alcohols. This invention satisfies that need.
Chemoenzymatic synthesis is a preparative strategy which uses chemical and biological steps in a reaction sequence. The biocatalytic reactions convert one organic compound to another by the use of enzymes, either isolated or as part of a biological system. Biocatalysts are especially useful for the introduction of chiral specificity into a reaction series.
The major drawback of biocatalytic systems is an unpredictable substrate specificity due to the lack of knowledge concerning the steric requirements of the enzyme's active site. Further complications can arise from enzyme inhibition caused by either the substrate or product.
With regard to the instant invention, Applicant was faced with the task of preparing optically pure diols. These diols are not strong acid or bases; hence, as pointed out above, the resolution of a racemic mixture of such materials presents a difficult task. Furthermore, the molecules of interest have a hydrophilic group substituted on a carbon alpha to a phenyl group. Up to the present invention, it was not known whether lipase from Pseudomonas fluorescens (or a similar enzyme) would accept and convert an ester precursor of such diol in a hydrolysis reaction. Applicant discovered that the enzyme would operate on such a substrate; this discovery was wholly unpredictable from the prior art.
Furthermore, Applicant discovered that the product of the reaction did not poison the enzyme to an extent which would prevent its use in a viable resolution process. In addition, Applicant also discovered a set of reaction conditions which permitted preparation of the desired diols in high optical purity. Moreover, Applicant discovered an elegant chemical synthesis for the preparation of a racemic ester used in the reaction step in which the enzyme is employed. This synthesis step affords a nearly quantitative yield of the substrate in a form which does not require purification before subjecting it to the action of the enzyme. In view of these discoveries, it is believed that Applicant's chemoenzymatic synthesis is a significant advance in the art.